1. Field of the Invention
This invention relates to a wireless monitoring system which is allowed to monitor or investigate a condition of a place inappropriate for a living thing (e.g., human) to access easily and safely, such as an altitude area, a high-concentration hazardous chemical substance filling area, a radiation management area, a high electric power passing area, a high temperature area, a high pressure area, an area which must be kept a high-level hygiene environment.
Especially, this invention relates to a wireless monitoring system and a child station in which replacing a battery of an apparatus (e.g., a child station) which enables such monitoring or investigation is not required, and can be assured a reliability of the apparatus after the apparatus is located.
2. Description of the Related Art
A social infrastructure system such as an electric power supply, a water supply, a communication, etc., is getting more sophisticated and complex according to a requirement for heightened convenience by a user. Once the system does not operate normally, not only are many direct users caused inconvenience and damage, but also users using other systems, which are related complexly to each other and may be affected by the damage, also may be caused some kind of damage and inconvenience. Further, there is a possibility that a confusion of whole area or whole society is caused. Such a wide area blackout or a wide area toxic chemical substance pollution, which is reported frequently, has already been developed as a big social problem.
One method for solving the above problem is to investigate a condition of an apparatus which is pivotal (critical) in the social infrastructure system and a condition of an environment surrounding the apparatus.
When the social infrastructure system becomes non-operational, if it is possible to investigate which apparatus does not run normally, then the social infrastructure system can be recovered early, and a bad influence on other social infrastructure systems is minimized.
Additionally, if it is possible to investigate or monitor periodically the operating condition of each apparatus in the social infrastructure system and the condition surrounding the apparatus, the social infrastructure system can be prevented from being non-operational.
Nowadays, since users want an advanced service, the apparatus which is pivotal (critical) in the social infrastructure system is concentrated in an area where is isolated and far from a usual residential area of the users, and the apparatus around the usual residential area is operated in an extreme environment having a dangerousness or a problem.
With regard to the electric power supply, there are a nuclear power station having a risk of a radiation exposure, a thermal power station having a risk of a flash fire explosion, a hydroelectric power station including many drops which have a long distance in elevation, and a transmission and transformation substation which transmits and distributes a high voltage electric power generated in an electric power station. With regard to the water supply, in light of safety and health, for preventing a reservoir and a water treatment plant from pollution by a living thing (e.g., human), easy access to these facilities is not allowed. With regard to communication, a base-station antenna is established at an altitude for providing a wireless communication, and a transmission media (e.g., optical fiber) is established on the ground or underground for providing an optical communication. Thus, in these cases, it is not easy to access these facilities.
In view of the above, focusing on a permeability, a diffraction propagation characteristic, and a non-contact characteristic of an electromagnetic wave, an attempt to add any detecting mechanism to the facilities and detect a condition of the facilities as a wireless signal by using a electric wave has been done.
However, a damping characteristic of the electromagnetic wave with regard to distance, according to a propagation characteristic of the electromagnetic wave-like spherical wave, is much larger than that of a cable communication in which an energy of a transmitting signal is confined in a cable and transmitted. For this reason, there is a problem of how to provide an electric power for radiating the electromagnetic wave to an extra space to transmit an information wirelessly to the apparatus constituting the social infrastructure system.
Although the apparatus uses the electric power in itself, applying a part of the electric power used by the apparatus to investigate an environment surrounding the apparatus causes a primary movement of the apparatus to give a disturbance and is avoided preferably.
Although a battery can be used, applying a primary battery is not preferred since a replacement of the primary battery is needed and may cause a new danger from the environment surrounding the apparatus. Moreover, although a secondary battery could be used, an assurance for a chemical characteristic of the secondary battery is not solved. Thus, considering a lifetime of an acceptable social infrastructure system, the secondary battery is not semi-permanent, so that the secondary battery cannot be a definitive solution.
As a solution without the battery, a method of providing the electric power used by the apparatus wirelessly could be employed. With this method, the problem of safety with regard to the battery replacement and a problem of a reliability may be solved, since a lifetime of a current electric element or electronic element is sufficiently long compared to the lifetime of the social infrastructure system.
However, since the electric power of the electromagnetic wave is reduced greatly during a space propagation as discussed previously, achieving an efficient signal transmission and an energy transmission by using the electromagnetic wave is a big technical problem.
As conventional technology to achieve transmission of the electric power and the signal by using the electromagnetic wave, some methods in which a wireless apparatus includes an antenna for transmitting an electric power wirelessly and an antenna for transmitting a signal are proposed. For example, in a system in which an apparatus located in a forest records a flight history of an airplane, a technology superposing the record on the electromagnetic wave and transmitting the record to exterior wirelessly is disclosed in “Microwave Theory and Techniques, IEEE Transactions; Vol. 53, Issue.12; p. 3735-3743” (non-patent document 1).
Further, as another conventional technology to achieve a transmission of the electric power and the signal by using the electromagnetic wave, some methods commonly called a “high-frequency wireless tag system” are proposed. For example, a system identifying a baggage by using the high-frequency wireless tag system is disclosed in “IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION; Vol. 53, No. 12; p. 3870-3876” (non-patent document 2).
However, in the technology disclosed in the non-patent document 1, there are some problems that (1) a location of the apparatus cannot be identified, since the apparatus is located in a forest, (2) since the apparatus executes a complex transaction for varying a direction of the electromagnetic wave radiated by the apparatus dynamically, the apparatus includes three antennas which are an antenna for transmitting a signal, an antenna for receiving a signal, and an antenna for obtaining the electric power, and a size of the apparatus becomes large, and (3) the apparatus needs extra electric energy in addition to the electric energy provided by the electromagnetic wave, because of the complex processing.
On the other hand, in the technology disclosed in the non-patent document 2, an antenna for transmitting and receiving a signal and an antenna for obtaining an electric power are provided as the same antenna, and the apparatus is downsized. However, since only a few meters are considered as a maximum distance between a base station and a child station (i.e., a mobile station) in this system, there is a problem that a distance for isolating the apparatus cannot be ensured sufficiently, when the child station is located at a place where there is not a good influence for a user.
Further, since the antenna for transmitting and receiving a signal and the antenna for obtaining an electric power is provided in one antenna in this system, and since communicating and obtaining an electric energy are operated with time division multiplex individually, a long time is needed for obtaining the electric energy to increase an energy of electromagnetic wave for extending a reachable distance of the electromagnetic wave radiated by the apparatus. An entire communication is not available during the obtaining the electric energy. For this reason, there is a problem that it is difficult for the base station to distinguish whether the child station is not available to communicate, or is obtaining the electric energy in a normal condition. Even though a complete signal transmission cannot be achieved, if at least an information cannot be provided regarding whether a communication link between the base station and the child station is established, it is difficult to ensure the reliability required by the current social infrastructure system.
Thus, prior to the present invention, there has been no conventional system or method which can provide a wireless monitoring system and a child station used therein which can monitor or investigate with high reliability a condition which is an operating condition of each apparatus located in a difficult-to-access environment and located in a social infrastructure system, or a condition of the environment where the apparatus is located, while the apparatus does not include a battery as a power source therein and is kept at a distance to isolate a user (human) from an influence of an environment (e.g., place) where the apparatus (e.g., child station) is located.